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Future nuclear power US scientists have made an important milestone in the costly, decades-old quest to develop fusion energy, which, if harnessed successfully, promises a nearly inexhaustible energy source for future generations.

For the first time, experiments have produced more energy from fusion reactions than the amount of energy put into the fusion fuel, say scientists at the Lawrence Livermore National Laboratory in California.

The researchers, led by physicist Dr Omar Hurricane, described the achievement as important, but said much more work is needed before fusion can become a viable energy source. They note that it did not produce self-heating nuclear fusion, known as ignition, which would be needed for any fusion power plant.

Researchers have faced daunting scientific and engineering challenges in trying to develop controlled nuclear fusion - the same process that powers stars including our Sun.

"Really for the first time anywhere, we've gotten more energy out of this fuel than was put into the fuel. And that's quite unique. And that's kind of a major turning point, in a lot of our minds," says Hurricane.

"I think a lot of people are jazzed."

Fusing together

Unlike fossil fuels or the fission process in nuclear power plants, fusion offers the prospect of abundant energy without pollution, radioactive waste or greenhouse gases.

Unlike the current nuclear fission energy that is derived from splitting atoms, fusion energy is produced by fusing atoms together.

Experts believe it still will be many years or decades before fusion can become a practical energy source.

"I wish I could put a date on it," says Hurricane. "But it really is (just) research. And, you know, although we're doing pretty good, we'd be lying to you if we told you a date."

Of the uncertain path ahead in fusion research, Hurricane compared it to "climbing half way up a mountain, but the top of the mountain is hidden in clouds. You can't see it. You don't have a map".

The research was conducted at the laboratory's National Ignition Facility (NIF), which was completed in 2009.

Zap a tiny target

The scientists used 192 laser beams to zap a tiny target containing a capsule less than 2 millimetres in diameter filled with fusion fuel, consisting of a plasma of deuterium and tritium, which are two isotopes, or forms, of hydrogen.

The fuel was coated on the inside of the capsule in a frozen layer less than the width of a human hair.

At very high temperatures, the nucleus of the deuterium and the nucleus of the tritium fuse, a neutron and something known as an "alpha particle" emerge, and energy is released.

The experiments, published in the journal Nature, created conditions up to three times the density of the Sun.

In two experiments described by the researchers that took place in September and November of last year, more energy came out of the fusion fuel than was deposited into it, but it was still less than the total amount deposited into the target.

The deuterium-tritium implosions were more stable than previously achieved. The researchers did so by doubling the laser power earlier in the laser pulse than in previous attempts.

The fusion-energy yield was increased by about ten-fold from past experiments, in a series that started last May. One of the experiments produced more than half of the so-called Lawson criteria needed to reach ignition — but only about one-100th of the energy needed for ignition.

Payoff could be big

Eager to exploit the potential this type of energy offers to reduce dependence on oil and other fossil fuels, the United States and other nations have invested many millions of dollars into fusion research, often with uneven results.

Steve Cowley, director of the Culham Centre for Fusion Energy, which is the British national laboratory for fusion research, called the new findings "truly excellent" but says different measures of success make it hard to compare with his type of research.

"We have waited 60 years to get close to controlled fusion, and we are now close in both magnetic and inertial confinement research. We must keep at it," says Cowley.

Mark Herrmann, a fusion researcher at Sandia National Laboratories in New Mexico, calls the new findings important, but sees a "very long road to assessing the viability of fusion as a long-term energy source".

"I believe a compact carbon-free energy source is very important for humankind in the long term," he says.